Total maternal weight gain at delivery has been consistently associated with both birth weight and perinatal mortality, thus current guidelines for prenatal care emphasize the importance of an adequate maternal weight gain during pregnancy. However, because total weight gain is not available prior to delivery, clinicians depend instead on the pattern of maternal weight gain that can be measured during the course of a pregnancy. Although this approach is practical, and is presumed to be useful in predicting poor pregnancy outcomes, there has been much less investigation of pattern of weight gain during pregnancy than the weight gain at delivery. We propose to study more than 16,000 pregnancies delivered since 1980 to identify how the pattern of maternal weight gain, summarized by a series of parametric models, predicts pregnancy outcome. Multivariate models will be used to explore the influence of the pattern of maternal weight gain on fetal birth weight, preterm delivery, and other outcomes, with special attention will be focused on subgroups of pregnant women who are at particularly high risk for poor fetal outcome. Finally, we will define and test an optimal pattern of weight gain during pregnancy that can be used in the clinical setting to identify individual pregnancies that are more likely to progress normally, as opposed to those which are likely to develop problems. It is expected that this study will contribute to the advice women are given about weight gain during pregnancy, to the effectiveness of monitoring prenatal weight gain and ultimately to improvement of health of newborn infants and their mothers.